var EARTH_RADIUS = 6378137.0;    //单位M
var PI = Math.PI;

function getRad(d){
    return d*PI/180.0;
}
function getFlatternDistance(lat1, lng1, lat2, lng2) {
	var f = getRad((lat1 + lat2) / 2);
	var g = getRad((lat1 - lat2) / 2);
	var l = getRad((lng1 - lng2) / 2);

	var sg = Math.sin(g);
	var sl = Math.sin(l);
	var sf = Math.sin(f);

	var s, c, w, r, d, h1, h2;
	var a = EARTH_RADIUS;
	var fl = 1 / 298.257;

	sg = sg * sg;
	sl = sl * sl;
	sf = sf * sf;

	s = sg * (1 - sl) + (1 - sf) * sl;
	c = (1 - sg) * (1 - sl) + sf * sl;

	w = Math.atan(Math.sqrt(s / c));
	r = Math.sqrt(s * c) / w;
	d = 2 * w * a;
	h1 = (3 * r - 1) / 2 / c;
	h2 = (3 * r + 1) / 2 / s;

	return d * (1 + fl * (h1 * sf * (1 - sg) - h2 * (1 - sf) * sg));
}